1. Field of the Invention
The present invention relates to semiconductor integrated circuit (IC) chips which can be tailored to include a fuse. The invention further relates to a method of making the improved circuit.
2. Related Art
Laser deletion of thick metal fuses is difficult due to the mass of metal that must be removed without damage to surrounding and underlying structures.
In the manufacture of semiconductor integrated circuits, wiring layers are deposited and defined and interconnected with conductive vias through a series of well known photolithography and metal etching steps. Each such wiring level is coated with a layer of a glassy protective material, known as a passivation layer, which protects and insulates the wiring of each layer. The creation of integrated circuits with such multiple wiring layers is well known to the semiconductor art.
In some circuits, such as CMOS logic circuits, the fuses designed in the circuit are often formed in regular arrays in the upper most layers of wiring and in a position such that other wiring is not placed immediately over or under the fuses. In such arrays the fuses are often aligned in parallel rows and placed as closely together as is possible. By opening selected ones of these fuses the logic elements of the circuits can be arranged in different combinations to perform different logic functions or correct manufacturing defects.
These fuses are typically opened by applying a laser pulse of sufficient size, duration and power as to superheat and vaporize the metal forming the fuse. This superheating of the fuse and its vaporization fractures and blows away a portion of the overlying glassy protective layer creating a saucer shaped crater in the protective layer. When the protective layer ruptures, cracks can radiate outwardly causing additional damage such as breakage of, or the uncovering of, adjacent fuse elements. Such uncovering of the adjacent elements can cause subsequent corrosion and premature failure of the circuit. While fuses are typically opened using a laser, they may also be opened by passage of electrical current or exposure to an ion beam which ablates (or removes or sputters) away the fuse link.
The reader is referred to the following patents related to fuses including "Array Protection Devices and Fabrication Method," U.S. Pat. No. 5,523,253, and "Array Fuse Damage Protection Devices and Fabrication Method," U.S. Pat. No. 5,420,455, both to Richard A. Gilmour, et al. and of common assignee to this invention, the contents of which are incorporated herein by reference in their entireties.
Fuses are used in semiconductor chips to provide redundancy, electrical chip identification and customization of function. For designs having three (or more) layers of wiring, the fuses are typically formed from a segment of one of the wiring layers, e.g., the "last metal" (LM) or "last metal minus one" (LM-1) wiring layer. Fusing, i.e., the deletion of a segment of metal fuse line, is accomplished by exposing the segment of metal fuse line to a short, high intensity pulse of "light" from an infra-red (IR) laser. The metal line absorbs energy, melts and expands, and ruptures any overlain passivation. The molten metal then boils or vaporizes out of its oxide surroundings, disrupting line continuity and causing high electrical resistance. A "sensing" circuit is used to detect fuse segment resistance.
Laser deletion of thick metal fuses is difficult due to the mass of metal that must be removed without damage to surrounding structures. As the mass of the fuse link increases or the melting temperature of the fuse link metal increases, higher laser energies and longer (or multiple) laser pulses are required to accomplish deletion. Higher energies and longer pulses provide sufficient energy to adjacent and underlying structures, e.g., silicon under the fuse area, to cause severe damage to the interlayer dielectric (ILD) oxide and adjacent fuse wiring. What is needed is a way to eliminate the need to use high laser energies.